The invention relates to the use of 1,3-substituted imidazolium salts of the formula I
in whichR1 and R3 independently of one another are an organic radical having 1 to 20 C atoms,R2, R4, and R5 independently of one another are an H atom or an organic radical having 1 to 20 C atoms, it also being possible for R4 and R5 together to form an aliphatic or aromatic ring, andX is a thiocyanate anionas latent catalysts for curing compositions comprising epoxy compounds.
Epoxy compounds are used for producing coatings, as adhesives, for producing shaped articles, and for numerous other purposes. In these applications they are generally present during processing in a liquid form (as solutions in suitable solvents or as liquid, solvent-free 100% systems). The epoxy compounds are generally of low molecular mass. In the context of the use they are cured. Various possibilities are known for curing. Starting from epoxy compounds having at least two epoxy groups it is possible, with an amino compound or with an acid anhydride compound having at least two amino groups or at least one anhydride group, respectively, for curing to take place through a polyaddition reaction (chain extension). Amino compounds or acid anhydride compounds of high reactivity are generally not added until shortly before the desired curing. The systems in question are therefore referred to as two-component (2K) systems.
Additionally it is possible to use catalysts for the homopolymerization or copolymerization of the epoxy compounds. Known catalysts include catalysts which are only active at high temperatures (latent catalysts). Latent catalysts of this kind have the advantage of allowing one-component (1K) systems; in other words, the epoxy compounds can comprise the latent catalysts without any unwanted premature curing occurring.
Latent catalysts available commercially include, in particular, adducts of boron trifluoride with amines (BF3-monoethylamine), quaternary phosphonium compounds, and dicyandiamide (DICY).
Journal of Polymer Science: Polymer Letters Edition, vol. 21, 633-638 (1983) describes the use of 1,3-dialkylimidazolium salts for this purpose. Their decomposition above 175° C. releases 1-alkylimidazoles, which then bring about the cure. The structure of the cation has been varied, and the halides chloride and iodide have been used as anions.
DE-A 2416408 discloses imidazolium borates, such as imidazolium tetraphenylborate or imidazolium tetra-n-butylborate.
U.S. Pat. No. 3,635,894 describes 1,3-dialkylimidazolium salts with anions selected from chlorides, bromides, and iodides as latent catalysts for epoxy compounds.
Kowalczyk and Spychaj, Polimery (Warsaw, Poland) (2003), 48(11/12), 833-835 describe the use of 1-butyl-3-methylimidazolium tetrafluoroborate as a latent catalyst for epoxy compounds. The activity of the catalyst does not begin until 190° C.
Sun, Zhang and Wong, Journal of Adhesion Science and Technology (2004), 18(1), 109-121 disclose the use of 1-ethyl-3-methylimidazolium hexafluorophosphate as a latent catalyst. The activity begins only at 196° C.
JP 2004217859 uses imidazolium tetraalkylborates or imidazolium dialkyldithiocarbamates. The activation takes place by exposure to high-energy light.
EP 0 458 502 discloses a multiplicity of very different catalysts for epoxy compounds. Included in the list are 1-ethyl-2,3-dimethylimidazolium acetate (R1=ethyl, R2=methyl, and R3=methyl in formula I) and 1-ethyl-2,3-dimethylimidazolium acetate-acetic acid complex.
Suitable latent catalysts ought to be readily miscible with the epoxy compounds. The mixtures ought to remain stable for as long as possible at room temperature under standard storage conditions, so that they are suitable as storable 1K systems. In the context of the use, however, the temperatures required for curing should not be excessively high, and in particular should be well below around 200° C. Lower curing temperatures allow energy costs to be saved and unwanted secondary reactions avoided. In spite of the lower curing temperature, the mechanical and performance properties of the cured systems ought as far as possible not to be impaired. The desire is that these properties (examples being hardness, flexibility, bond strength, etc) should be at least as good and if possible even better.
An object of the present invention, therefore, were imidazolium salts as latent catalysts, and mixtures of these imidazolium salts and epoxy compounds meeting the requirements set out above.